Carbon nanotubes are ideal structures for the tips used in scanning probe microscopy, such as atomic force microscopy (AFM), since carbon nanotubes (i) have an intrinsically small diameter, which is comparable to that of molecules in the case of single walled nanotubes (SWNT), (ii) have a high aspect ratios, (iii) can buckle elastically, and (iv) can be selectively modified at their ends with organic and biological species to create functional probes. In the past, most nanotube probe tips have been made by mechanical attachment of multi-walled nanotube (MWNT) and SWNT bundles to silicon tips in optical or electron microscopes. Nanotube tips made in this way have been used to demonstrate, for example, their potential for high-resolution and chemically-sensitive imaging, but also highlighted limitations. Specially, mechanical tip fabrication is a time consuming one-by-one process, and the resolution of tips can vary widely due to their bundle structures. More recently, direct catalytic growth of nanotubes from conventional tips has been explored, which demonstrated that individual multi-walled nanotubes (MWNT) could be grown by CVD from the ends of Si tips with controlled orientation. In this growth method of nanotube probes, commercial AFM tips were selectively etched to create nanopores, which were created at the apex of silicon tips by chemical etching or focused ion beam milling, to guide the growth of nanotube probes in an orientation ideal for imaging. Electrochemically or electrophoretically deposited iron in the nanopores was used to catalyze the selective CVD growth of nanotubes with an orientation controlled by the pores. Tips synthesized using the electrochemically deposited iron catalyst were shown to consist reproducibly of individual 3-5 nm radii MWNTs oriented optimally for high-resolution imaging. Significantly, these studies demonstrated that a well-defined synthetic approach could be used to prepare directly nanotube probes, thus opening the possibility of precise control over nanotube size and thereby tip resolution. Recently, SWNT tips having much smaller radii of only 1-3 nm were reproducibly grown using well-defined iron oxide nanocluster catalysts These latter tips begin to approach the theoretical minimum size expected for individual SWNTs.
The pore-growth method has demonstrated the potential of CVD to grow directly controlled diameter nanotube tips, although it still has limitations. In particular, the preparation of nanopores can be time-consuming and may not place individual SWNTs at the optimal location on the flattened apex. It would therefore be desirable to develop a reliable growth method for SWNTs that eliminates the need for nanopores.
Scanning probe microscopes (SPMs), such as the scanning tunneling microscope (STM) and atomic force microscope (AFM), are now widely used for these purposes with the capability of working at length scales as small as a single atom. However, the single probe tips employed in SPMs limit these tools in manipulation and the measurement of physical properties; for example, a single tip cannot grab an object, and a second electrical contact must be made to structures for electrical measurements. Two probes in the form of a tweezers could overcome these limitations of SPMs, and thus may enable new types of fabrication and facile electrical measurements on nanostructures.
Micrometer scale electromechanical tweezers, which represent basic micro-electromechanical systems, employing tungsten as a tip material, have previously been fabricated on silicon. Tungsten deposition and subsequent processing were used to produce 200 xcexcm long by 2.5 xcexcm wide tungsten arms that could be closed by applying a potential (V) of ca. 150 volts and then opened again by reducing V to zero. The potential difference between the tungsten arms of the tweezers produces an attractive electrostatic force that can overcome the elastic restoring force of the beams in closing the tweezers. Smaller tweezers with 30 xcexcm long by 0.25 xcexcm long single crystal silicon arms, which responds at a potential of 45 V, have also been fabricated using conventional lithography and processing steps. Such micro-tweezers, if removed from the substrate support, could be useful tools in manipulation. However, due to their relatively large size and large actuating voltages these tweezers are not suitable for work in the nanometer regime.
In one aspect of this invention provides for a method of producing a carbon nanotube tip, comprising the steps of: providing a tip assembly; applying a metallic catalytic material to the tip assembly; inserting said metallic catalytic material bearing probe into a CVD reactor; and exposing said metallic catalytic material bearing probe to a gaseous atmosphere comprising a carbon containing gas thereby producing a tip bearing a carbon nanotube tip. In an embodiment, the tip assembly comprises silicon.
In another embodiment, the tip assembly is a multifaced probe. In another embodiment, one or more faces of the tip assembly comprises a mask. In an even further embodiment, the mask is removable. In one embodiment, the multifaced probe comprises silicon.
In another embodiment, the method produces a carbon nanotube tips on an array of tip assemblies.
In another emodiment, the metallic catalytic material is selected from the group consisting of metals, metal oxides, metallic salts, metallic particles and metallic colloids. In a further embodiment, the metallic catalytic material is selected from the group consisting of iron salts, nickel salts, cobalt salts, platinum salts, molybdenum salts, and ruthenium salts. In a further embodiment, the metallic catalytic material is selected from the group consisting of iron colloids, nickel colloids, cobalt colloids, platinum colloids, molybdenum colloids, and ruthenium colloids. In an even further embodiment, the metallic catalytic material is a ferric salt. In another embodiment, the metallic catalytic material is ferric nitrate. In another embodimetn, the metallic catalytic material is an iron colloid
In an further embodiment, the metallic catalytic material is in solution. In another embodiment, the solution comprises an alcohol. In a further embodimetn, the alcohol is selected from the group consisting of methanol, ethanol and isopropanol.
In another embodiment, the carbon containing gas is ethylene. In yet another embodiment, the carbon nanotube tip is a SWNT . In another embodiment, the carbon nanotube tip is a bundle of SWNT. In another embodiment, the carbon nanotube tip is a MWNT.
In a further embodiment, the steps of producing a carbon nanotube tip comprises the step of shortening the carbon nanotube tip by electrical etching. In an even further embodiment, electrical etching comprises applying voltage pulses of a predetermined voltage between the nanotube tip and a support surface.
In another aspect, the invention provides for a method of fabricating nanotube-based nanostructures by controlled deposition of nanotube segments of a nanotube tip, comprising the steps of: biasing the nanotube tip at a starting location on a substrate at a predetermined voltage; scanning the tip along a predetermined path; and applying a voltage pulse at a higher voltage than the predetermined voltage to disconnect the tip from the nanotube segment on the substrate. In a further embodiment, the nanotube tip is a single wall nanotube.
In another aspect, the invention provides for a method of producing nano tweezers made of carbon nanotube tips, comprising the steps of: providing a support surface; applying at least two independent electrodes to the support surface; and applying at least one carbon nanotube tip on each of the electrodes, wherein the spacing between respective end portions of the carbon nanotube tips changes in response to a voltage applied between the at least two electrodes. In a further embodiment, applying at least one carbon nanotube tip comprises the steps of: applying metallic catalytic material to the at least one electrode; and inserting said electrode into a CVD reactor; and exposing said electrode to a gaseous atmosphere comprising a carbon containing gas, thereby producing a tip bearing a carbon nanotube tip. In a further embodiment, the carbon nanotube tip is a single SWNT. In a further embodiment, wherein the carbon nanotube tip is a bundle of SWNTs. In a further embodiment, wherein the carbon nanotube tip is a MWNT.
Further features and advantages of the present invention will be apparent from the following description of preferred embodiments and from the claims.